What is an ichthyosaur? The real sea dragons that once swam beneath the waves

For centuries, the remains of prehistoric sea creatures have inspired extraordinary myths and legends.

One such group are the ichthyosaurs, whose remains have become part of sea dragon mythology. Thanks to the work of people such as Mary Anning, we now know that these animals weren’t legends, but real reptiles that lived millions of years ago.

While the ichthyosaurs are now much less mysterious than they once were, there’s still a lot we don’t know about this diverse group of animals. Some grew to join the ranks of the largest animals that ever lived, while others would have looked similar to dolphins as they dashed through the water.

Though they’re now long extinct, the ichthyosaurs have changed how we understand the world.

What are ichthyosaurs?

Ichthyosaurs are a group of ancient marine reptiles whose ancestors returned to the sea more than 250 million years ago. Though they’re both reptiles and lived at the same time, dinosaurs aren’t closely related to ichthyosaurs. The dinosaurs lived on land, whereas ichthyosaurs never left the water.

As they adapted to life in the ocean, ichthyosaurs evolved a more streamlined shape similar to that of fish and marine mammals through a process known as convergent evolution.

The ichthyosaur skeleton’s similarity with other animals made it hard for early scientists to work out what they were. For example, Edward Lhwyd identified them as fish when he published the first known illustrations of ichthyosaur vertebrae in 1699. Other scientists, however, thought that they might be ancient crocodiles or dolphins.

This only began to change in 1811, when a 15-year-old boy called Joseph Anning stumbled upon an ichthyosaur skull near Lyme Regis on the coast of southern England. A year later, his sister Mary returned to the same site and began to excavate the rest of the skeleton, beginning her career as a famed fossil collector, dealer and palaeontologist.

By early 1813, this work had paid off. Mary Anning’s ichthyosaur was the first largely complete specimen to be uncovered. It caused a sensation after it was studied by Sir Everard Home in 1814. Dr Marc Jones, our Curator of Fossil Reptiles and Amphibians, says that the discovery of ichthyosaurs and other marine reptiles transformed our understanding of life on Earth.

“Studying the fossils of ichthyosaurs, plesiosaurs and other animals made scientists consider the possibility of extinction,” Marc explains. “They showed that Earth was very old and had experienced a lot of changes, such as sea level rise and climate change, so that not every animal that had lived was still alive.”

“Their distinctive fossils also captured the public’s interest, leading to early palaeoart and demand for these specimens to be displayed in museums for everyone to see.”

Sir Everard continued to study ichthyosaurs and, after studying another of Mary’s finds in 1819, finally gave these animals the name Proteosaurus. However, this name didn’t catch on.

Instead, scientists started using Ichthyosaurus, a name coined by our former Keeper of Mineralogy Charles Konig two years earlier in 1817. Ichthyosaurus, meaning ‘fish reptile’, ultimately inspired the name of the entire group.

At the time, all ichthyosaur specimens were thought to be Ichthyosaurus fossils. Scientists Henry de la Beche and William Conybeare, for example, named the first three Ichthyosaurus species I. communis, I. platyodon and I. tenuirostris.

However, as more bones were discovered, scientists eventually realised that there were many kinds of ichthyosaurs. I. platyodon, for example, is now known as Temnodontosaurus and I. tenuirostris as Leptonectes.

Further discoveries have shown that skulls, teeth and limb structure vary a lot in ichthyosaurs, leading to many more species being named. This extra evidence has also helped to reveal even more about how these animals lived.

How did ichthyosaurs evolve?

While ichthyosaurs have been known about for more than 200 years, their exact origins remain a mystery. Dr Aubrey Roberts, our Scientific Associate, says that a lack of early ichthyosaur fossils makes it difficult to understand where they came from.

“At the moment, the ancestors of ichthyosaurs are thought to be a group of land-living reptiles that adapted to the sea,” Aubrey explains. “Unfortunately, we haven’t found any of the fossils that show this transition, so we don’t know exactly what reptiles ichthyosaurs evolved from.”

“It’s likely that ichthyosaur ancestors were diapsids – the group containing everything from dinosaurs to crocodiles – but their exact place in the family tree is the subject of ongoing scientific debate.”

Until recently, it was thought that ichthyosaurs evolved in the aftermath of the end-Permian mass extinction around 252 million years ago, when around 95% of all species were wiped out. However, Aubrey’s research on the Arctic Svalbard archipelago suggests that ichthyosaurs evolved before then.

“On Svalbard, we’ve been studying a section of rocks that spans from right at the end of the Permian to the Early Triassic,” explains Aubrey. “These contain bone-rich layers that we’ve dated to around 250 million years ago that contain fossils of coelacanths, crocodile-like marine amphibians and the earliest known ichthyosaurs.”

“These fossil ichthyosaurs are already well adapted to life in the ocean, suggesting they’ve already been evolving for some time. This suggests that ichthyosaurs actually come from the Late Permian more than 252 million years ago and managed to live through the mass extinction that followed.”

If this is the case, it would explain why scientists looking for ichthyosaur ancestors haven’t found them in Triassic rocks – they were never there to start with. Instead, looking for these fossils in Late Permian rocks from lagoons and shallow seas may finally reveal ichthyosaurs transitioning from land to water.

Where did ichthyosaurs live?

Without knowing exactly when and how ichthyosaurs first evolved, it’s hard to know exactly where they came from. This task is made even more complicated by the fact that ichthyosaurs were able to rapidly spread around the world as soon as they became capable of swimming.

Despite this, there are clues that suggest that ichthyosaurs first evolved in Asia. For example, some of the closest known relatives of ichthyosaurs are a group of unusual marine reptiles from China known as the hupehsuchians.

These are toothless animals with long, thin bodies, armour plating on their back and webbed flippers. At first, this makes them seem quite different to the ichthyosaurs. However, closer examination reveals that they’re linked by similarities in their backbones and limbs.

Regardless of where they evolved, ichthyosaurs began to spread across large parts of the world after the end-Permian mass extinction. With their competitors driven to extinction, Triassic ichthyosaurs soon adapted to fill many different roles.

“One of the most distinctive early ichthyosaurs is Cymbospondylus,” Aubrey says. “It was a top predator, growing to as much as 17 metres in length and with sharp teeth that it could use to eat fish and other marine reptiles.”

“Other early ichthyosaurs have more rounded teeth, suggesting that they crushed and ate shelled animals such as ammonoids. Then there are more generalist species that would have eaten a wide range of fish, squid and other animals.”

The adaptability of the ichthyosaurs meant that they subsequently spread across all the oceans of the world. This would have been made easier by the fact that they never had to return to land to reproduce or give birth. Fossils of pregnant ichthyosaurs show that they gave birth to live young in the open ocean.

As a result, fossils of these animals are found everywhere from the Arctic to Antarctica. It’s estimated that there are more than 100 species of ichthyosaur in total, with many more yet to be studied.

However, there’s one habitat that ichthyosaurs weren’t able to conquer. While other marine reptiles are known to have adapted to life in freshwater, there’s no strong evidence so far that ichthyosaurs ever lived in lakes and rivers.

One of the few possible signs of freshwater ichthyosaurs comes from a fossil found near Swanage in southern England. While it’s uncertain exactly where this ichthyosaur is from, research suggests that it came from a largely freshwater lagoon that was distantly connected to the sea.

If this is the case, our understanding of ichthyosaurs would be transformed – demonstrating the ongoing importance of UK fossils for understanding these animals.

UK ichthyosaurs

Perhaps the most famous site for ichthyosaur fossils in the UK is the Jurassic Coast, a stretch of coastline covering around 150 kilometres of southern England. This is where Mary Anning found her ichthyosaurs, and it’s still a rich source of fossils today.

People walking along its beaches might stumble across an ichthyosaur tooth or Ichthyosaurus vertebrae that have eroded out of the cliffs and ended up lying on the beach.

Other excavations are more complex, with teams of experts and specialist equipment needed to take well-preserved skeletons away. Many of these specimens end up in museums, where they’re studied by scientists such as Aubrey and Marc.

“Ichthyosaur remains are well-known from the UK,” says Marc. “While the Jurassic Coast is probably the best-known region for these fossils, ichthyosaurs are also regularly found along the coastlines of northern Yorkshire and Somerset.”

“Though it’s more common to find ichthyosaur fossils on the coast, where rocks are constantly eroded by the sea, some are found far inland. This is because these areas used to be part of the seabed millions of years ago.”

One famous example of an inland ichthyosaur is known as the Rutland ichthyosaur, Rutland Sea Dragon or, more formally, as the species Temnodontosaurus trigonodon. It was found in Rutland Water, in the East Midlands, around 60 kilometres from the coast, during maintenance work on the reservoir in 2021.

The 10-metre-long animal is currently the largest ichthyosaur ever found in the UK, and there are plans for it to go on display in a museum near to where it was discovered. However, more incomplete fossils from other parts of the UK suggest even bigger ichthyosaurs once lived in the surrounding oceans.

Ichthyosaur size

The largest ichthyosaur is Ichthyotitan severnensis, a species known from jaw fossils discovered in Somerset, UK. While the rest of the skeleton is missing, comparisons of the jaw bones with other ichthyosaurs suggest that Ichthyotitan’s size was as much as 25 metres long.

Not only would this make Ichthyotitan the biggest ichthyosaur ever known, but also the largest marine reptile as well. It’s also one of the biggest ever animals, measuring around the same length as the average blue whale.

Ichthyotitan is thought to be the last known member of a wider group of giant ichthyosaurs known as the shastasaurids. These species existed for around 30 million years until the end of the Late Triassic, 201 million years ago, and have been found in Asia, Europe and North America.

Though no other shastasaurid was as big as Ichthyotitan, species such as Shastasaurus and Shonisaurus still measured between 15 and 21 metres long. Their huge size meant that they would have been an important part of life in Late Triassic oceans.

It’s likely they would have played a similar role in the marine ecosystem to whales today, helping to cycle nutrients and supporting rich communities of deep-sea animals after their death.

Ichthyosaur lifestyle

It wasn’t just shastasaurids that were ocean predators. All known ichthyosaurs were carnivores, with preserved faeces and stomach contents showing that they ate a variety of different animals ranging from small fish and squid to other marine reptiles.

Some ichthyosaur prey were truly enormous. A five-metre-long Guizhouichthyosaurus skeleton found in China contains the remains of a crocodile-like thalattosaur that was almost as big as the Guizhouichthyosaurus itself. It’s possible that this ichthyosaur might have bitten off more than it could chew, as it died shortly after swallowing this meal.

Other ichthyosaurs might have been divers, hunting for prey hundreds of metres beneath the surface. Species such as Ophthalmosaurus have some of the largest eyes in the animal kingdom, which are thought to have helped them to see in the darkness.

They would have been able to survive in the deep sea because all ichthyosaur species are thought to have been warm blooded. An exceptionally well-preserved fossil from Germany also suggests that ichthyosaurs had blubber to help trap their body heat, similar to modern dolphins.

Some ichthyosaurs, especially a group known as the parvipelvians, would actually have looked surprisingly similar to these marine mammals. Scientists can tell this because the outlines of an ichthyosaur’s body, and its soft tissue, can sometimes be preserved along with its skeleton.

The first ichthyosaur soft tissue fossils were discovered around the 1840s, but it wasn’t until the 1890s that more complete specimens were found. German scientist Eberhard Fraas reported on a well-preserved ichthyosaur that showed that these animals had a dorsal fin in the middle of their back.

“Preserved soft tissue from the fins and flippers has given us a better idea about how these animals moved,” Marc adds. “Preserved skin has also been found, telling us more about their appearance.”

“Unlike many other reptiles, we know that ichthyosaurs had smooth skin to help them be more streamlined underwater. We’ve even found skin with evidence of pigmentation, which shows what colour ichthyosaurs were.”

While some ichthyosaurs might have had completely black skin, others appear to have been counter-shaded. This means that they had dark skin on the top of their bodies and lighter skin underneath to help them blend into the ocean.

Both techniques would have helped to hide ichthyosaurs from their prey. A giant ichthyosaur soft tissue fossil from a Temnodontosaurus suggests that some species appear to have had ridges and structures similar to winglets on their flippers that helped them move more quietly underwater.

These adaptations for stealth would also hinder any animals trying to eat them. Ichthyosaur predators would have included plesiosaurs, sharks and other ichthyosaurs, based on tooth marks left behind on their fossils.

Ichthyosaur extinction

The extinction of the ichthyosaurs didn’t happen all in one go but was a gradual process that happened over more than 100 million years. Different groups went extinct at different times as the pressures of a changing world eventually overwhelmed them.

At the end of the Triassic, for example, ichthyosaurs faced a series of smaller extinction events that ended in a mass extinction 201 million years ago. Major volcanic eruptions caused the temperature and acidity of the ocean to increase, making it a much more difficult place to live.

Many smaller marine species that ichthyosaurs fed on, such as the conodonts, were wiped out in this extinction. While smaller ichthyosaurs were able to survive, the giant shastasaurids disappeared – possibly because there wasn’t enough food to support them.

The extinction of the shastasaurids led to new opportunities for the parvipelvians, which had only evolved in the Late Triassic. They diversified into a variety of species in the Jurassic, including Ichthyosaurus, Temnodontosaurus and many others.

Many of these ichthyosaurs subsequently went extinct at the end of the Jurassic, around 145 million years ago. The causes of this extinction are still shrouded in mystery, but it had a particularly devastating impact on the oceans. For many years, it was thought that it pushed ichthyosaurs into a long-term decline that led to their eventual extinction.

However, more recent discoveries show that many ichthyosaurs survived the end of the Jurassic. Malawania, a relative of Ichthyosaurus found in Iraq, reveals that this family of ichthyosaurs survived despite disappearing from the fossil record for more than 60 million years.

Other ichthyosaurs thrived, with a group known as the platypterygiines evolving many new species in the Cretaceous. Fossils of these animals are found across the world, showing that ichthyosaurs were still a recognisable part of life in the oceans.

Around 90 million years ago, however, the ichthyosaurs finally faced an extinction they couldn’t survive. Undersea volcanic eruptions caused oxygen levels in the ocean to drop dramatically, while temperatures in the tropics reached 35°C or more.

It’s not clear why, having survived so many extinctions before, that this was the time that the ichthyosaurs died out. It’s possible that the multiple rounds of extinction had left the remaining species vulnerable to competition from other marine predators or that they were unable to find a refuge from the changes in the environment.

“Another possibility is that accumulated environmental changes over the existence of the ichthyosaurs finally caught up with them,” says Marc. “Animals can only adapt to challenges that face them at the time, so the evolutionary changes they’d previously experienced to survive might have hampered their ability to cope with the changes in the Cretaceous.”

The final extinction of the ichthyosaurs left a void in the oceans that was filled by other marine reptiles, including plesiosaurs and mosasaurs. These animals lived on for another 24 million years until Earth was struck by a giant asteroid, ending the era of the dinosaurs and these marine reptiles once and for all.

This article includes information from the book Ancient Sea Reptiles: Plesiosaurs, Ichthyosaurs, Mosasaurs & More by Dr Darren Naish.